1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to self-servo writing a disk drive by propagating interleaved sets of timing clocks and servo bursts during alternate time intervals.
2. Description of the Prior Art
Disk drives for computer systems comprise a disk for storing data and a head actuated radially over the disk for writing data to and reading data from the disk. To effectuate the radial positioning of the head over the disk, the head is connected to the distal end of an arm which is rotated about a pivot by a rotary actuator (e.g., a voice coil motor (VCM)). The disk is typically divided into a number of concentric, radially spaced tracks, where each track is divided into a number of data sectors. The disk is typically accessed a data sector at a time by positioning the head over the track which comprises the target data sector. As the disk spins, the head writes transitions (e.g., magnetic transitions) in the data sector to record data, and during read operations senses the transitions to recover the recorded data.
Accurate reproduction of the recorded data requires the head to be positioned very close to the centerline of the target data sector during both write and read operations. Thus, accessing a target data sector involves positioning or xe2x80x9cseekingxe2x80x9d the head to the target track, and then maintaining centerline xe2x80x9ctrackingxe2x80x9d while data is written to or read from the disk. A closed loop servo system typically performs the seeking and tracking operations by controlling the rotary actuator in response to position information generated from the head.
A well known technique for generating the head position control information is to record servo information in servo sectors disbursed circumferentially about the disk, xe2x80x9cembeddedxe2x80x9d with the data sectors. This is illustrated in FIG. 1 which shows a disk 2 comprising a number of concentric tracks 4 and a number of embedded servo sectors 6. Each servo sector 6 comprises a preamble 8, a sync mark 10, servo data 12, and servo bursts 14. The preamble 8 comprises a periodic pattern which allows proper gain adjustment and timing synchronization of the read signal, and the sync mark 10 comprises a special pattern for symbol synchronizing to the servo data 12. The servo data 12 comprises identification information, such as sector identification data and a track address. The servo control system reads the track address during seeks to derive a coarse position for the head with respect to the target track. The track addresses are recorded using a phase coherent Gray code so that the track addresses can be accurately detected when the head is flying between tracks. The servo bursts 14 in the servo sectors 6 comprise groups of consecutive transitions (e.g., A, B, C and D bursts) which are recorded at precise intervals and offsets with respect to the track centerline. Fine head position control information is derived from the servo bursts 14 for use in centerline tracking while writing data to and reading data from the target track.
The embedded servo sectors 6 are written to the disk 2 as part of the manufacturing process. Conventionally, an external servo writer has been employed which writes the embedded servo sectors 6 to the disks by processing each head disk assembly (HDA) in an assembly line fashion. The external servo writers employ very precise head positioning mechanics, such as a laser interferometer, for positioning the head at precise radial locations with respect to previously servo-written tracks so as to achieve very high track densities.
There are certain drawbacks associated with using external servo writers to write the embedded servo sectors 6 during manufacturing. Namely, the HDA is typically exposed to the environment through apertures which allow access to the disk drive""s actuator arm and the insertion of a clock head which requires the servo writing procedure to take place in a clean room. Further, the manufacturing throughput is limited by the number of servo writers available, and the cost of each servo writer and clean room becomes very expensive to duplicate.
Attempts to overcome these drawbacks include a xe2x80x9cself-servo writingxe2x80x9d technique wherein components internal to the disk drive are employed to perform the servo writing function. Self-servo writing does not require a clean room since the embedded servo sectors are written by the disk drive after the HDA has been sealed. Further, self-servo writing can be carried out autonomously within each disk drive, thereby obviating the expensive external servo writer stations.
U.S. Pat. No. 5,949,603 as well as IBM Technical Disclosure Bulletin, Vol. 33, No. 5 (October 1990) disclose in an article entitled xe2x80x9cRegenerative Clock Technique for Servo Track Writesxe2x80x9d a technique for self-servo writing wherein the servo sectors are written relative to clock data disbursed around the disk and propagated from track to track. The clock data is first written to a xe2x80x9cseedxe2x80x9d track (e.g., at the inner diameter of the disk) from which the clock data as well as the servo sectors are propagated to the remaining tracks. The head is positioned over the seed track and, while reading the clock data in the seed track, the head is moved away from the seed track until the amplitude of the read signal decreases to some predetermined level. Then the clock data and servo sectors are written to the first track adjacent to the seed track. This process is repeated for the next and subsequent tracks until the embedded servo sectors have been written over the entire surface of the disk. Because the head cannot read and write simultaneously, the clock data is propagated in even and odd interleaves. When servo writing a current track, the even clock data from a previously servo-written track is read to derive timing and head position control information while writing the odd clock data and servo sectors to the current track. When servo writing the next track, the odd clock data from the previously servo-written track is read to derive timing and head position control information while writing the even clock data and servo sectors to the next track, and so on.
This process is illustrated in FIG. 2A and FIG. 2B. FIG. 2A shows a read element being offset from a current track until the amplitude of the read signal decreases to a predetermined level while reading the A (even) clock data. Thereafter a write element begins writing the B (odd) clock data and servo sectors (SS) for the next track. Notice that the head never simultaneously reads and writes because writing to the next track occurs between the A clock data. When finished writing the B clock data and servo sectors to the track, the head is offset from the finished track until the read signal decreases to the predetermined level while reading the just-written B clock data as illustrated in FIG. 2B. The write element then writes the A clock data and the servo sectors to the next track.
When self-servo writing the disk it is important to maintain proper spacing between adjacent tracks as well as proper alignment of the servo sectors from track to track so as to achieve a high recording density (tracks per inch) as well as preserve the phase-coherent nature of the Gray code in the track addresses and proper alignment of the servo bursts. Thus, it is important for the head to maintain the desired radial offset from the previously servo-written track while writing the servo sectors to a current track. In this respect, the above-described prior art self-servo writing technique suffers because the clock data is used both for radial positioning of the head as well as circumferential timing. Various system dynamics induce noise in the read signal while reading the clock data (e.g., errors in writing the clock data, media defects, electronic noise, etc.) which translates into errors when generating the head position control information for use in maintaining the desired radial offset from the previously servo-written track while writing the servo sectors to a current track.
There is, therefore, a need for a self-servo writing technique which provides a more accurate estimate of the head position control information used to maintain proper tracking while writing the servo sectors to the disk. In particular, there is a need to improve upon the prior art technique of generating the head position control information based on maintaining a predetermined amplitude level in the read signal.
The present invention may be regarded as a self-servo writing disk drive comprising a disk having a first radial location and a second radial location, the first radial location for storing first A clock data interleaved with first B clock data and first A servo bursts interleaved with first B servo bursts. The second radial location for storing second A clock data interleaved with second B clock data and second A servo bursts interleaved with second B servo bursts. The disk drive further comprises a head having a read element and a write element, wherein the read element is radially offset from the write element. A memory stores a self-servo writing program which is executed by a control system. During a first time interval, the read element is positioned over a region of the first radial location. The read element then reads at least part of the first A clock data to generate first circumferential timing information and at least part of the first A servo bursts to generate first position control information. The first position control information is used for positioning the write element over the second radial location. The first circumferential timing information is used for writing at least part of the second B clock data and at least part of the second B servo bursts to the second radial location. During a second time interval, the read element is positioned over the region of the first radial location. The read element then reads at least part of the first B clock data to generate second circumferential timing information and at least part of the first B servo bursts to generate second position control information. The second position control information is used for positioning the write element over the second radial location. The second circumferential timing information is used for writing at least part of the second A clock data and at least part of the second A servo bursts to the second radial location.
The present invention may also be regarded as a method of self-servo writing a disk drive comprising a disk having a first radial location and a second radial location, the first radial location for storing first A clock data interleaved with first B clock data and first A servo bursts interleaved with first B servo bursts, the second radial location for storing second A clock data interleaved with second B clock data and second A servo bursts interleaved with second B servo bursts. The disk drive further comprises a head having a read element and a write element, wherein the read element is radially offset from the write element. During a first time interval, the read element is positioned over a region of the first radial location. The read element then reads at least part of the first A clock data to generate first circumferential timing information and at least part of the first A servo bursts to generate first position control information. The first position control information is used for positioning the write element over the second radial location. The first circumferential timing information is used for writing at least part of the second B clock data and at least part of the second B servo bursts to the second radial location. During a second time interval, the read element is positioned over the region of the first radial location. The read element then reads at least part of the first B clock data to generate second circumferential timing information and at least part of the first B servo bursts to generate second position control information. The second position control information is used for positioning the write element over the second radial location. The second circumferential timing information is used for writing at least part of the second A clock data and at least part of the second A servo bursts to the second radial location.